Over the past decade, the technology has been intensively developed for fabricating flat panel displays (FPDs), such as used for computer displays and more recently for television screens. Sputtering is the preferred approach in fabricating flat panels for depositing onto large generally rectangular panels of glass or polymeric panels or flexible sheets electrically conductive layers including metals such as aluminum and molybdenum and transparent conductors such as conductive metal oxides such as indium tin oxide (ITO). The completed panel may incorporate thin-film transistors, plasma displays, field emitters, liquid crystal display (LCD) elements, or organic light emitting diodes (OLEDs). Solar cells having p-n or p-i-n junctions may be similarly formed on low-cost substrates. Similar technology may be used for coating glass windows with optical layers or forming color filters on FPDs. It may also be used for fabricating solar cells, especially on lower-cost substrates. Flat panel sputtering is principally distinguished from the long developed technology of wafer sputtering by the large size of the substrates and their rectangular shape. Demaray et al. describe such a flat panel sputter reactor in U.S. Pat. No. 5,565,071, incorporated herein by reference in its entirety. Their reactor includes, as illustrated in the schematic cross section of FIG. 1, a rectangularly shaped sputtering pedestal electrode 12, which is typically electrically grounded, for holding a rectangular glass panel 14 or other substrate in opposition to a rectangular sputtering target 16 within a vacuum chamber 18. The target 16, at least the surface of which is composed of a metal to be sputtered, is vacuum sealed to the vacuum chamber 18 across an isolator 20. Typically, a layer of the material to be sputtered is bonded to a backing plate in which cooling water channels are formed to cool the target 16. A sputtering gas, typically argon, is supplied into the vacuum chamber 18 held at a pressure in the milliTorr range. Advantageously, a back chamber 22 is vacuum sealed to the back of the target 16 and vacuum pumped to a low pressure, thereby substantially eliminating the pressure differential across the target 16 and its backing plate. Thereby, the target assembly can be made much thinner. When a negative DC bias is applied to the conductive target 16 with respect to the pedestal electrode 12 or other grounded parts of the chamber such as wall shields, the argon is ionized into a plasma. The positive argon ions are attracted to the target 16 and sputter metal atoms from it. The metal atoms are partially directed to the panel 14 and deposit thereon a layer at least partially composed of the target metal. Metal oxide or nitride may be deposited in a process called reactive sputtering by additionally supplying oxygen or nitrogen into the chamber 18 during sputtering of the metal.
To increase the sputtering rate, a linear magnetron 24, also illustrated in schematic bottom view in FIG. 2, is conventionally placed in back of the target 16. It has a central pole 26 of one vertical magnetic polarity surrounded by an outer pole 28 of the opposite polarity to project a magnetic field within the chamber 18 and parallel to the front face of the target 16. The two poles 26, 28 are separated by a substantially constant gap 30 over which a high-density plasma is formed in the chamber 18 under the correct chamber conditions and flows in a close loop or track. The outer pole 28 consists of two straight portions 32 connected by two semi-circular arc portions 34. The magnetic field traps electrons and thereby increases the density of the plasma and as a result increases the sputtering rate of the target 16. The relatively small widths of the linear magnetron 24 and of the gap 30 produces a higher magnetic flux density. The closed shape of the magnetic field distribution along a single closed track forms a plasma loop generally following the gap 30 and prevents the plasma from leaking out the ends. However, the small size of the magnetron 24 relative to the target 16 requires that the magnetron 24 be linearly and reciprocally scanned across the back of the target 16 in a direction transverse to the long dimension of the linear magnetron 24. Typically, a lead screw mechanism drives the linear scan, as disclosed by Halsey et al. in U.S. Pat. No. 5,855,744 in the context of a more complicated magnetron. Although horseshoe magnets may be used, the preferred structure includes a large number of strong cylindrical magnets, for example, of NdBFe arranged in the indicated pole shapes with their orientations inverted between the two indicated polarities. Magnetic pole pieces may cover the operating faces to define the pole surfaces and a magnetic yoke bridging the two poles 26, 28 may magnetically couple the other sides of the magnets.
De Bosscher et al. have described a coupled two-dimensional scan of such a linear magnetron in U.S. Pat. Nos. 6,322,679 and 6,416,639.
The described magnetron was originally developed for rectangular panels having a size of about 400 mm×600 mm. However, over the years, the panel sizes have continued to increase, both for economy of scale and to provide larger display screens. Reactors are being developed to sputter onto panels having a size of about 2 m×2 m. One generation processes a panel having a size of 1.87 m×2.2 m and is called 40K because its total area is greater than 40,000 cm2. A follow-on generation called 50K has a size of greater than 2 m on each side. The invention however can be practiced for solar cells especially when the substrate is not a glass panel but other, more economical substrates including a rolled substrate having parts sequentially presented to the sputtering apparatus. The widths of linear magnetrons are generally constrained to be relatively narrow if they are to produce a high magnetic field. As a result, for larger panels having minimum dimensions of greater than 1.8 m, linear magnetrons become increasingly ineffective and require longer deposition periods to uniformly sputter the larger targets and coat the larger substrates.
In one method of accommodating larger targets, the racetrack magnetron 24 of FIG. 2 is replicated up to nine time in the transverse direction along the scanning direction to cover a substantial portion of the target. See U.S. Pat. No. 5,458,759 to Hosokawa et al. Scanning is still desired to average out the magnetic field distribution. However, there are several disadvantages to this replication approach. First, the separated magnetrons are not believed to optimally utilize the magnetic fields of the constituent magnets. That is, the effective magnetic field is less than is possible. Secondly, a significant number of particles have been observed to be produced during striking of the plasma at the portions of the magnetron near to the plasma dark space shields, which are adjacent to the arc portions 34 of the outer pole 28 of the racetrack magnetron 24. It is believed that electrons leak from the plasma to the nearby shield. Striking voltages of about 800 VDC are required. Such high voltages are believed to disadvantageously produce excessive particles. Thirdly, the prior art using one racetrack magnetron 24 of FIG. 2 reciprocally scans the magnetron at a relatively high speed over a large fraction of the target size to perform approximately 30 to 40 scans during a typical one minute sputter deposition period. Such high scanning rates require a difficult mechanical design for the much heavier magnetrons covering a substantial fraction of the larger target. Fourthly, scanning magnetrons including one or more racetrack magnetrons do not completely solve the uniformity problem. The lateral edge portions of the target 16 underlying the ends of the racetrack magnetron 24 receive a high time-integrated magnetic flux because the arc portions 34 extend in large part along the scan direction. Also, the axial edge portions of the target underlying the magnetron when the scan direction reverses also receive a high time-integrated magnetic flux because of the finite time need to reverse directions. Thus, the target edges are disproportionately eroded, reducing the target utilization and target lifetime, as well as contributing to non-uniform deposition.